I presume that you are talking about a thermistor/diode on a wire that you need to glue down somewhere. In theory, to get a reasonable temperature from the processor, this thermistor/diode needs to be between the processor and its heatsink, but doing so will disaffect the thermal transfer between the processor and the heatsink, so I strongly recommend that you DO NOT do this. The alternative is to place it on the surface of the heatsink (one of its fins) as near to the processor as possible, but I doubt you will get a good reading out of it. These types of displays are just not practical anymore...
All modern processors come with a thermal sensor *inside* the chip itself. It will obviously give the most possible accurate reading, better than any probe you might stick to it.
The value of this temperature can be read by the software included with the motherboard.
I suggest you use the probe you have to test the air temperature inside the case, as I do, it is quite a useful information anyway.
For that purpose the best place is near the fan blowing air out of the case.